üySEOtextüz0. General Formulation, [0] Action Principle, (1) Action, An
action is defined as such a functional I that the classical equation of
motion is derived from vanishing of variation of I. â┬I(q)/â┬qi(t)=0, â┬I(âË)/â┬âËâ┐(x)=0
(2) Symmetry, If I is invariant under an operator U, that is: I(U(âË))=I(âË),
then the equation of motion is invariant under U. â┬I(âË)/â┬âËâ┐(x)=0ü╠â┬I(U(âË))/â┬[U(âË)]â┐(x)=0